I. Field of the Invention
This invention relates to a toner concentration control device for controlling the concentration of a toner in a powdery developing agent as used in an elctrophotographic copying machine so that it is maintained constant.
II. Description of the Prior Art
In an electrophotographic copying machine using a developing agent comprising a toner and carrier, "fogging" occurs when the concentration of a toner in the developing agent becomes higher than a predetermined value. This is because the toner is deposited onto the white portion of an image. Furthermore, there sometimes occurs the case where a mix of the toner and carrier ceases to function as a developing agent when used for a prolonged time period. When, on the other hand, the concentration of the toner is lowered an image as a whole becomes light or pale, failing to faithfully reproduce an original image. For this reason, it is necessary to maintain always constant the concentration of a toner in the developing agent.
U.S. Pat. No. 3,897,748 discloses a method in which an amount of fresh toner corresponding to an amount of toner spent by development is supplied into a developing agent. In this method, a developer (a toner/carrier mix) after development is contacted with a rotating cylindrical probe, made of an electroconductive material, to cause the toner and carrier to be separated from each other to permit only the toner to be picked up by the probe. An amount of toner picked up by the probe corresponds to the concentration of the toner in the developing agent after development.
The amount of toner picked up by the probe is detected by an optical means and an amount of fresh toner corresponding to the amount of toner spent by development is supplied by a toner supply device. According to the method of this patent the concentration of the toner in the developing agent can be automatically maintained always constant.
In this method, however, the toner pickup probe has its surface mirror-finished and it is cumbersome to manufacture. Moreover, it is difficult to maintain the surface state of the probe constant over a longer period of usage. As a result, an amount of toner picked up by the probe does not correspond to the concentration of the toner in the developing agent. It is thus difficult to always maintain a stable and accurate toner concentration.